Generally, a thermal recording material has a substrate and a heat-sensitive recording layer that is formed thereon and that contains, as main components, a generally colorless or light-colored electron-donating dye precursor and an electron-accepting developer. When the thermal recording material is heated with a thermal head, a hot pen or a laser beam, the dye precursor and the electron-accepting developer readily react with each other to give a colored image. Such thermal recording materials are used in broad fields of measuring recorders, facsimile machines, POS printers, ATM/CD, handy terminals, labeling machines, automatic vending machines of railway tickets, and the like, owing to advantages that recording apparatuses for them are so simple that their maintenance is easy and that they make no noise.
In recent years, thermal recording apparatus have made progress in downsizing, high-speed printing and power consumption reduction, and a thermal recording material is required to have an excellent “anti-sticking property”. “Sticking” refers to a series of phenomena that occur since a thermal head and a thermal recording material stick to each other during printing under heat with the thermal head, and it specifically refers to an abnormal noise made during printing, non-printing spot(s) in the form of white streak(s), printed characters that are shrunken in the printing paper feeding direction, and the like. It degrades the continuous printability and printing quality that should originally be smooth. When a recording apparatus is poor in the strength of feeding a recording sheet or in particular when printing is practiced in a low-temperature environment, the sticking is liable to take place. For improving thermal recording materials in the anti-sticking property, therefore, there is a method in which a lubricant such as a fatty acid metal salt or waxes are incorporated (e.g., see JP 50-30539A) or a method in which an oil-absorbing pigment is incorporated (e.g., see JP 53-86299A). In these methods, it is difficult to attain a sufficient anti-sticking property with a recording apparatus that is poor in the strength of feeding a recording sheet or in a low-temperature printing environment.
Further, as thermal recording materials have come to be used in various fields, it is required to improve “image stability”, i.e., to maintain a ground whiteness and concurrently keep a color from vanishing. A method of incorporating a diphenyl sulfones-bridged type compound is effective as a method of achieving image stability (for example, see JP10-297089A and JP10-297090A). However, it is known that this method entails a decrease in anti-sticking property, and even if the above lubricant or oil-absorbing pigment is incorporated, this method has a slight effect on the improvement of anti-sticking property. It is hence demanded to improve the anti-sticking property sufficiently.
It is essential to satisfy both the anti-sticking property and the image stability at high levels, and there are a number of proposals to form a protective layer on a thermal recording layer, and there are found methods of incorporating a specific pigment (for example, see JP62-53879A, JP9-142026A and JP2002-86911A). In these proposals, however, the protective layer per se hampers the efficiency of color development, and there is hence a great decrease in coloring sensitivity. In particular, when the printing speed of a printing apparatus is increased or when the power consumption is reduced by decreasing printing energy, the print density is liable to be decreased. Further, when a thermal recording material has a protective layer, the number of steps of production thereof is larger than that of a counterpart having no protective layer, and the production cost thereof is higher.